According to the general technology, one transmitting antenna and one receiving antenna have been used. Multiple-Input Multiple-Output (MIMO) means a scheme that uses a plurality of transmitting antennas and a plurality of receiving antennas. Transmission and reception efficiency can be improved by the MIMO scheme. Namely, a transmitting end or receiving end of a wireless communication system can enhance capacity and improve throughput by using a plurality of antennas. Hereinafter, MIMO may be referred to as ‘MIMO antenna’.
The MIMO antenna technology does not depend on a signal antenna path to receive a whole message. Instead, in the MIMO antenna technology, data fragments received from a plurality of antennas are incorporated to complete data. If the MIMO antenna technology is used, a data transmission rate can be improved within a specific sized cell region, or system coverage can be enhanced with a specific data transmission rate. Also, the MIMO antenna technology can widely be used for a user equipment for mobile communication and a relay node. According to the MIMO antenna technology, it is possible to overcome limitation of a transmission rate in mobile communication according to the related art where a single antenna is used.
A schematic view of a general MIMO communication system is illustrated in FIG. 1. Referring to FIG. 1, NT number of transmitting antennas are provided at a transmitting end while NR number of receiving antennas are provided at a receiving end. If a plurality of antennas are used at both the transmitting end and the receiving end, theoretical channel transmission capacity is more increased than that a plurality of antennas are used at any one of the transmitting end and the receiving end. Increase of the channel transmission capacity is proportional to the number of antennas. Accordingly, the transmission rate is improved, and frequency efficiency is also improved. Supposing that a maximum transmission rate is Ro when a single antenna is used, a transmission rate corresponding to a case where multiple antennas are used can be increased theoretically as much as a value obtained by multiplying a maximum transmission rate Ro by a rate increase Ri.
For example, in a MIMO communication system that uses four transmitting antennas and four receiving antennas, a transmission rate four times greater than that of a single antenna system can be obtained. After such theoretical capacity increase of the MIMO system has been proved in the middle of 1990, various technologies have been actively studied to substantially improve a data transmission rate. Some of the technologies have been already reflected in the standard of various wireless communications such as third generation mobile communication and next generation wireless LAN.
The MIMO antenna technology can be divided into a spatial diversity scheme and a spatial multiplexing scheme, wherein the spatial diversity scheme is to enhance transmission reliability using symbols which have passed through various channel paths, and the spatial multiplexing scheme is to increase a transmission rate by simultaneously transmitting a plurality of data symbols using a plurality of transmitting antennas. Also, a hybrid scheme of the above two schemes has been provided to appropriately obtain advantages of the two schemes.
In respect of the aforementioned MIMO technology, active studies are ongoing in view of various aspects such as the study of information theoretical aspect related to MIMO communication capacity calculation under various channel environments and multiple access environments, the study of radio channel measurement and model of a MIMO system, and the study of time space signal processing technology for improvement of transmission reliability and transmission rate. In particular, a study of a method for efficiently controlling an uplink transmission power of a data channel in consideration of a transmission rank will be required.